The present invention relates to a two-way optical fiber transmission network for efficient transmission of information between subscribers scattered around a center.
Active research efforts are now being made in the industry to develop a method of two-way optical fiber transmission between subscribers which introduces optical fibers into subscribers' line to achieve high-speed, broadband transmission of information including video-service. The outline of such transmission system is given, for instance, in a paper titled "Fiber Optic Subscriber Communication System-Overview", Kenkyu Jitsuyoka Houkoku (E.C.L. Tech. Jour.), NTT, Japan, Vol. 34, No. 7, pp. 1049-1056, written by S. Shimada. FIG. 6 is a functional block diagram showing one example of conventional communication system. This communication system connects the center 1a and a subscriber 2a through one optical fiber cable 3 through which video information, telephone data and high-speed data are transmitted in both directions by using light signals with two different wavelengths (.lambda.1,.lambda.2).
To describe in more detail, in the center 1a, information signals from video selecting equipment, broadband switching system and digital local switching system are converted into light signals with wavelength .lambda.1 by optical transmitter 5-1 from which they are transferred to optical multi/demultiplexer 4-1 and further transmitted over optical fiber cable 3 to the subscriber 2a. Having reached the subscriber 2a, the light signals with wavelength .lambda.1 are demultiplexed and fed to optical receiver 6-1 where they are converted into electric signals which are then supplied to television sets 7-1, 7-1, telephone 7-3 and facsimile 7-4.
Conversely, information from the subscriber 2a such as from television 7-5, telephone 7-6 and facsimile 7-7 is transferred to optical transmitter 5-2 from which it is converted into light signals with wavelength .lambda.2. The converted light signals are further transferred to optical multi/demultiplexer 4-2 and transmitted over optical fiber cable 3 to the center 1a where they are demultiplexed by optical multi/demultiplexer 4-1, fed to optical receiver which converts the received optical signals into electric signals that are then supplied to the switching system.
The subscribers, as shown in FIG. 7, are widely distributed around the center 1 as represented by areas A1 through A4. The distances from the center to the areas l1-l4 greatly vary from several hundred meters at minimum in the case of area A4 to 10 kilometers at maximum for area A1. The subscriber's population also varies from area to area; some area has a large subscriber population and some area has a very small number of subscribers.
Such regional differences in the distance from the center, however, result in differences in the optical fiber cable loss, as shown in FIG. 8. That is, received signal power varies from one subscriber to another. In the case of a single mode optical fiber using a wavelength of 1.3 .mu.m of FIG. 8, for example, there is a received signal power difference of approximately 18 dB between subscribers located 10 kilometers and 200 meters from the center (see FIG. 9). This requires installing in the receivers of the center and subscribers an AGC (automatic gain control) circuit that produces a dynamic range of more than 20 dB or a variable optical attenuator. This disadvantage not only increases the system's cost but lowers the transmission efficiency.